1. Field of the Invention
The invention relates to displays particularly with respect to liquid crystal, half-tone displays. Such displays typically are of active matrix configuration.
2. Description of the Prior Art
Backlighted liquid crystal displays (LCD) utilizing twisted-nematic (TN) liquid crystal have been developed to provide flat panel displays for applications such as aircraft instrumentation, laptop and notebook computers, and the like. Such LCDs typically utilize a rear electrode structure in the form of a matrix of transparent metal pixels or dot electrodes and a continuous transparent metal front electrode with the liquid crystal material sandwiched therebetween. The front electrode is often denoted as the common or counter electrode. Each pixel electrode is activated through a switch, usually implemented as a thin film transistor (TFT), which is deposited as a field effect transistor (FET). The drain electrode of each TFT is connected to, or actually forms, the pixel electrode with which it is associated. The gate electrodes of the TFTs in each row of the matrix are commonly connected to a gate bus-line for the row and the source electrodes of the TFTs in each column of the matrix are commonly connected to a source bus-line for the column. An image is created in raster fashion by sequentially scanning the gate bus rows while applying information signals to the source bus columns.
Color capability is imparted to the LCD in a well known manner by providing suitable color filters at the front surface of the LCD to intercept the light transmitted through the respective pixels and appropriately addressing the pixels to display the desired colors. For example, delta shaped triads with primary color RED, GREEN and BLUE filters are often utilized. By appropriate video control of the gate and source buses, various colors are generated. In the conventional design, each pixel electrode has a storage capacitor connected thereto for supplementing the charge holding capacity of the pixel so as to retain the voltage on the pixel between refresh pulses.
As an improvement to the conventional display pixel described above, half-tone pixels are utilized to impart gray scale capability to the LCD. A particularly desirable half-tone panel arrangement is described in U.S. Pat. Nos. 4,840,460 and 5,126,865. Said U.S. Pat. No. 4,840,460, issued Jun. 20, 1989, is entitled "Apparatus And Method For Providing A Gray Scale Capability In A Liquid Crystal Display Unit". Said U.S. Pat. No. 5,126,865, issued Jun. 30, 1992, is entitled "Liquid Crystal Display With Sub-pixels". Said U.S. Pat. Nos. 4,840,460 and 5,126,865 are assigned to the Assignee of the present invention and are incorporated herein by reference in their entirety.
Briefly, a half-tone pixel is comprised of multiple sub-pixels with the activating TFT connected to a first one of the sub-pixels and with the remaining sub-pixels deriving their activation voltage from the TFT through respective coupling capacitors. As described in said U.S. Pat. Nos. 4,840,460 and 5,126,865, sub-pixels are selectively activated by controlling the activation voltage from the TFT to provide the gray scale capability. Conventionally, one or more of the half-tone sub-pixels has a storage capacitor associated therewith for the reasons described above with respect to the conventional pixel.
Although a half-tone pixel is often comprised of a first or primary sub-pixel with plural secondary sub-pixels, the following descriptions will be provided in terms of a liquid crystal display dot with two sub-pixels. The TFT activating voltage is applied directly to the primary sub-pixel and the second sub-pixel derives its activation voltage through its associated coupling capacitor. It is appreciated that the concepts and structures described herein with respect to the second sub-pixel also apply to the other secondary sub-pixels.
In the half-tone panel technology, the second sub-pixel voltage is derived from the first sub-pixel voltage as: EQU V.sub.2 =V.sub.1 [C.sub.c /(C.sub.c +C.sub.s2 +C.sub.lc2)] (1)
where:
V.sub.1 =the first sub-pixel voltage PA1 V.sub.2 =the second sub-pixel voltage PA1 C.sub.s2 =capacitance of second sub-pixel storage capacitor PA1 C.sub.c =capacitance of coupling capacitor PA1 C.sub.lc2 =capacitance of second sub-pixel.
A problem prevalent in the LCD half-tone display panel technology is achieving uniformity of gray levels across the panel. In order to provide acceptable gray level uniformity, the voltage relationship between the sub-pixels of a pixel should be independent of position on the panel. Quantitatively, V.sub.2 /V.sub.1 should be position independent. It is appreciated from the above, that V.sub.2 /V.sub.1 depends on C.sub.c, C.sub.lc2, and C.sub.s2. V.sub.1 is the primary voltage applied by the switches to the pixels and is substantially position independent over the panel. C.sub.lc2 is also reasonably well controlled and position independent over the panel so as to obtain uniformity of background. Thus, it is seen that the secondary voltage V.sub.2 is sensitive to variations in C.sub.c and C.sub.s2.
It is desirable to make the sub-pixel voltages V.sub.2 uniform over the panel. In the half-tone panel technology, however, the depositions that form C.sub.c and C.sub.s2 are not uniform and capacitances formed can vary as much as .+-.20% across the panel. The non-uniformity across the panel is exacerbated when, for example, either C.sub.c or C.sub.s2 is 20% larger and the other is 20% smaller. The variation is primarily caused by differences in dielectric thicknesses for the different depositions and the non-uniformity is exacerbated if C.sub.c is controlled by one deposition while C.sub.s2 is controlled by another deposition permitting the tolerances to accumulate.